1. Field of the Invention
The present invention relates to a method for preparing amides, and more particularly, to a method for separating an amide from an amino acid ionic liquid.
2. Description of the Prior Art
Caprolactam is an important raw material in the manufacture of nylon 6 fibers and thin films. Beckman rearrangement of cyclohexanone oxime is an important reaction step in producing caprolactam. Currently, oleum is used as a catalyst for converting cyclohexanone oxime to caprolactam sulfate during Beckman rearrangement, and then ammonia is used for neutralization, so as to obtain caprolactam. While the conversion rate of cyclohexanone oxime is almost 100% and the selectivity for caprolactam is 99%, a large amount of low-valued ammonium sulfate is generated during the reaction, and concentrated sulfuric acid used for catalysis causes problems such as corrosion to the whole equipment and environmental pollution. In the recent years, researches on new production technologies of caprolactam focus on reducing or avoiding the generation of the by-product, ammonium sulfate. Moreover, compared with the gas phase reaction, liquid-phase rearrangement has advantages including moderate reaction conditions, fewer requirements to the equipments, etc., and is advantageous to the reconstruction of the current equipments. As a result, scholars worldwide have put efforts on developing liquid-phase rearrangement, and attained substantial developments and breakthrough. For example, in Chinese Patent No. 1852898A assigned to Sumitomo Chemical Company Ltd. in Japan, an ionic liquid having the sulfonate group is used as a catalyst to give the selectivity of caprolactam up to 99%. In Chinese Patent No. 1919834 assigned to Lanzhou Institute of Chemical Physics in China, an ionic liquid having sulfuryl chloride is used as a catalyst to give the selectivity of caprolactam up to 97.2%. In WO2008/145312A1 assigned to DSM N.V. in Netherlands, an anionic solution having sulfate is used for conversion reaction to give the selectivity of amide up to 99%.
In addition, it has been disclosed that in the rearrangement reaction with oleum, sulfuric acid is neutralized with an alkaline, and then caprolactam is extracted by an organic solvent. For example, in U.S. Pat. Nos. 3,944,543, 4,036,830 and 3,694,433, an amide is extracted to an organic solvent by the solvent including an arylalkyl, haloakyl and alcohol, so as to be further purified. U.S. Pat. Nos. 4,328,154, 4,013,640 and 3,912,721 disclose extracting an amide by a solvent, alkylphenol. In U.S. Pat. No. 6,111,099, nylon 6 is depolymerized, caprolactam is recovered by extraction with alkyl phenolic compounds, wherein the alkyl is C6-25 alkyl, and the caprolactam and alkyl phenolic compounds are recovered by distillation.
In the recent years, due to the concepts of atom economy and environmental economy, ionic liquids are widely applied in academia and industry. Particularly, it is developed to use the ionic liquids are in a rearrangement reaction of cyclohexanone. For example, Chinese Patent No. 1670017 discloses that Beckman rearrangement is performed with an ionic liquid and a catalyst system including a phosphorus compound. However, the acidic phosphorus compounds are easily coupled to the products of the rearrangement reaction, such that the products of the rearrangement reaction are in the ionic liquid phase rather than in the organic solvent, resulting in very low efficiency of extraction by a solvent or vacuum distillation. Although the products can be separated by neutralization with ammonium, the addition of ammonium results in the formation of ammonium sulfate, which needs to be avoided in industry.
The above-mentioned techniques have high conversion rate of ketoximes and high selectivity of amides, but the rearrangement reaction with non-oleum is hard to be used in industry due to the above drawbacks. Specifically, caprolactam cannot be separated from the rearrangement reaction system. In other words, it is the barrier in eliminating byproduct ammonium sulfate that amides need to be separated from a liquid phase in Beckman rearrangement reaction.
Hence, it is a need to develop a method for separating products from Beckman rearrangement reaction and recycling the ionic liquid in industry.